Auto merge of #83207 - oli-obk:valtree2, r=lcnr

normalize mir::Constant differently from ty::Const in preparation for valtrees

Valtrees are unable to represent many kind of constant values (this is on purpose). For constants that are used at runtime, we do not need a valtree representation and can thus use a different form of evaluation. In order to make this explicit and less fragile, I added a `fold_constant` method to `TypeFolder` and implemented it for normalization. Normalization can now, when it wants to eagerly evaluate a constant, normalize `mir::Constant` directly into a `mir::ConstantKind::Val` instead of relying on the `ty::Const` evaluation.

In the future we can get rid of the `ty::Const` in there entirely and add our own `Unevaluated` variant to `mir::ConstantKind`. This would allow us to remove the `promoted` field from `ty::ConstKind::Unevaluated`, as promoteds can never occur in the type system.

cc `@rust-lang/wg-const-eval`

r? `@lcnr`

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -10,6 +10,7 @@ use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
+use rustc_middle::mir;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -214,4 +215,8 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
         let constant = constant.super_fold_with(self);
         constant.eval(self.infcx.tcx, self.param_env)
     }
+
+    fn fold_mir_const(&mut self, constant: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        constant.super_fold_with(self)
+    }
 }